This invention generally concerns nickel-base alloys and particularly concerns a castable and weldable nickel-base alloy having sufficient creep strength for use in gas turbine multi-vane nozzle applications. 2. Description of Prior Developments
Nickel-base alloy design involves adjusting the concentrations of certain critical alloying elements to achieve the desired mix of properties. For a high temperature alloy suitable for use in turbine nozzle applications, such properties include high temperature strength, corrosion resistance, castability and weldability. Unfortunately, by optimizing one property another property can often be adversely affected.
Alloy design is a compromise procedure which attempts to achieve the best overall mix of properties to satisfy the various requirements of component design. Rarely is any one property maximized. Rather, through development of a balanced chemistry and proper heat treatment, the best compromise among the desired properties is achieved.
An example of such compromise or trade-off is that between high-temperature alloys which are repair weldable or those which possess superior creep resistance. In general, the easier it is to weld a high-temperature alloy, the more difficult it is to establish satisfactory creep strength. This problem is particularly acute in the case of alloys for gas turbine applications. In addition to being repair weldable and creep resistant, gas turbine nozzle alloys should also be castable and highly resistant to low cycle fatigue, corrosion and oxidation.
Prior cobalt-based alloys have proved adequate for first stage turbine nozzle applications, notwithstanding their susceptibility to thermal fatigue cracking. The reason for the acceptance of these alloys is the ease with which they may be repair welded. However, in latter stage nozzles, cobalt-based alloys have been found to be creep limited to the point where downstream creep of the nozzles can result in unacceptable reductions of turbine diaphragm clearances. Although cobalt-based alloys with adequate creep strength for these latter stage nozzle applications are available, they do not possess the desired weldability characeristics.
While cast nickel-base alloys, as a group, possess much higher creep strengths than cobalt-base alloys, the nickel-base alloys have not generally been used in nozzle applications for heavy duty industrial gas turbines because of their well-known lack of weldability. In effect, conventional nickel-base alloys possess more creep strength than required for many turbine nozzle applications. An example of such an alloy is disclosed in U.S. Pat. No. 4,039,330. Although this nickel-base alloy possesses superior creep strength, its marginal weldability may complicate or prevent the repair of cracked turbine components by welding.
Another problem associated with using nickel-base alloys in gas turbine applications involving large investment castings is the possible detrimental effect on the physical metallurgy of the alloy which can be caused by elemental segregation. Elemental segregation occur during the relatively slow solidification of large castings at which time undesirable phases, such as eta phase, can be formed in the alloy, or can be caused to form during subsequent sustained high-temperature exposure. Since large turbine nozzle segments are subject to this condition, a carefully balanced mix of alloying elements must be maintained to avoid formation of such phases. When these phases are formed in amounts causing reductions in mechanical properties, the alloy is said to be metallurgically unstable.
Still another drawback of conventional nickel-base alloys is the often complicated and time-consuming heat treatments necessary to achieve desired end properties, which causes the cost of these alloys to be increased.
Accordingly, a need exists for a nickel-base alloy having the necessary creep strength for primary and latter stage turbine nozzle applications. This alloy, to be commercially feasible, should be castable and easy to weld in order to satisfy industry repair demands. Furthermore, such an alloy should be relatively quickly and economically heat treated and substantially immune to metallurgical instability. In addition, the alloy should possess superior resistance to corrosion and oxidation.